1. Field of the Invention
The present invention relates to a display device and a manufacturing method of the same.
2. Description of the Related Art
In recent years, development toward the practical use of an EL display, which uses an electroluminescence element (hereinafter referred to as an EL element) for a pixel portion, has been promoted. Especially an EL display using an organic EL element can be operated with a driving voltage equivalent to that of a liquid crystal element which is mainly used in conventional flat panel displays, and the driving voltage is lower than that of an inorganic EL element. Specifically, compared to a liquid crystal display, an EL display device using an organic EL element is self-luminous, thus does not require a backlight, and its color reproducibility is high. Therefore, it is highly expected to be a key technology to the next-generation flat panel display.
On the other hand, an organic EL element has a problem in that changes in the properties, such as a reversible change associated with an environmental temperature change and an irreversible change like element deterioration caused by moisture or the like, specifically, are larger than those of an inorganic EL element. In order to ensure a constant luminance characteristic with the sufficient life cycle in a wide range of usage environments, some kind of correcting means for such changes in the properties is required.
As an example of a correcting means for the aforementioned changes in the properties, a method of keeping the value of a current supplied to an organic EL element constant can be given. In an organic EL element, the relation between a voltage applied to an element and a current flowing in the element is nonlinear, but the relation between a current flowing in the element and the luminance of light emission of the element is almost linear. Therefore, the above-described method is advantageous since it is a relatively easy way to keep the luminance constant.
As an example of the correcting means, a method in which an organic EL element for monitoring the current value (hereinafter referred to as a monitoring element) is formed in the vicinity of an organic EL element that is formed as a pixel portion, and a power supply potential of a current supply line of the organic EL element is controlled such that the value of a current flowing to the monitoring element becomes constant is proposed (see Patent Document 1: Japanese Published Patent Application No. 2003-330419).